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Avoiding Scratched Windows
Scratches on the window can greatly reduce the performance of the imaging system. Motorola recommends
recessing the window into the housing or applying a scratch resistance coating.
Window Material
Many window materials that look clear can contain stresses and distortions that reduce performance. For this
reason, use only cell-cast plastics or optical glass (with or without an anti reflection coating, depending on the
application). Following are descriptions of three popular window materials: chemically tempered float glass,
PMMA, and ADC (CR-39
Suggested Window Properties
Table 2-2
Property
Material
Thickness
Wavefront Distortion (transmission)
Clear Aperture
Surface Quality
Chemically Tempered Float Glass
The preferred window material, glass is hard and provides excellent scratch and abrasion resistance. However,
unannealed glass is brittle. Increasing flexibility strength with minimal optical distortion requires chemical
tempering. Glass cannot be ultrasonically welded and is difficult to cut into odd shapes.
Plastics
Plastic materials are not recommended for tilted windows since surface scratches cause image artifacts.
Colored windows are not recommended if motion detection mode (e.g., presentation mode) is required since it
reduces engine sensitivity to the moving target. If using plastic is necessary, the following two materials are
available.
Cell Cast Acrylic (ASTM: PMMA)
Cell Cast Acrylic, or Poly-methyl Methacrylic (PMMA) is fabricated by casting acrylic between two precision
sheets of glass. This material has very good optical quality, reasonably good impact resistance and low initial
cost, but is relatively soft and susceptible to attack by chemicals, mechanical stresses, and UV light. Therefore
polysiloxane coating is strongly recommended. Acrylic can be laser cut into odd shapes and ultrasonically
welded.
Cell Cast ADC (ASTM: ADC)
Also known as CR-39
Most plastic eyeglasses sold today are uncoated, cell-cast CR-39. This material has excellent chemical and
environmental resistance, and reasonably good impact resistance. It also has quite good surface hardness,
and therefore does not have to be hard-coated, but may be coated for severe environments. This material
cannot be ultrasonically welded.
TM
).
Table 2-2
outlines the suggested window properties.
Clear cell-cast acrylic
0.06 in. (1.5 mm)
0.2 wavelengths peak-to-valley maximum and 0.04 λ maximum rms
over any 0.08 in. diameter within the clear aperture
To extend to within 0.04 in. of the edges all around
60-20 scratch/dig
TM
, Allyl Diglycol Carbonate (ADC) is a thermal-setting plastic produced by cell-casting.
Installation
Description
2 - 5
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